Management Methods: Biological Control

Impacts of BioControl

Ecological Basis of Biocontrol

The practice of biocontrol is based on two broad ecological cornerstones (Kok 1974, McEnvoy 1996).

Cornerstone #1: One organism can be used to control another organism

Biocontrol agents released into invasive plant populations must not only survive and multiply in their new environment, but they must also attack and cause enough damage to their host to cause significant population decline.

Biocontrol is used to reduce invasive plant populations to levels below damaging thresholds, not to eradicate plant species. Because biocontrol agents rely on sufficient host plant populations to provide their food and habitat, they will not completely eliminate their host plant populations.

Cornerstone #2: Some organisms have a limited host range

To reduce dominance of an invasive plant within the plant community, effective biocontrol agents must focus their actions upon the target plant without harming other vegetation. To be considered for release in the United States, insect biocontrol agents must feed and develop only on the target plant, and in some cases, only on a few closely related plant species. This is known as host-specificity and is the most important precondition for an insect to be used as a biocontrol agent. Researchers must demonstrate a biocontrol agent’s host-specificity in order receive a permit for importation into the United States, issued by the USDA Animal and Plant Health Inspection Service, Plant Protection and Quarantine (USDA-APHIS-PPQ).

Potential biocontrol agents often undergo five or more years of rigorous testing to ensure that host-specificity requirements are met. For example, surveys and host-specificity testing for potential biocontrol agents for purple loosestrife began in Europe in 1986. The first releases in the United States were made seven years later, following resolution of concerns surrounding nontarget feeding impacts on other plant species (Blossey 1997). One agent under consideration was not proposed for introduction because it exhibited a wider host range (Blossey and Schroeder 1995).

Effects of Biocontrol on Invasive Plants

Classical biocontrol agents are by definition "exotic, usually coevolved" natural enemies of the target invasive plant. The natural enemies of plants include herbivorous vertebrates (e.g., domestic livestock and fish) and invertebrates (e.g., insects, mites, and nematodes), fungal pathogens (e.g., rusts), and bacterial and viral diseases. Insects and mites are most commonly imported for classical biocontrol, whereas nematodes, diseases, and generalist herbivorous vertebrates are less common.

The level of mortality or injury inflicted on the host plant in its native environment is an important consideration when evaluating a natural enemy’s potential effectiveness as an imported biocontrol agent. Natural enemies can injure or kill their host plant directly, or can weaken the plant so that it is less competitive in its environment. Direct damage occurs when natural enemies suck out plant fluids, defoliate, eat seeds, and bore into roots and shoots. Insect enemies may indirectly affect their host by exposing the plant to secondary pathogen infections that can further weaken the plant (Wilson and McCaffrey 1999).

Some plants have developed mechanisms to counteract the action of the coevolved enemy (Trumble et al. 1993). Compensatory effects may occur when a plant damaged by a biocontrol agent increases relative growth rates and competitive effects, induces the production of chemicals that might harm neighboring plants, or stimulates the release of root exudates (i.e., Callaway et al. 1999).

Plant populations are typically regulated by a complex of natural enemies (Frick 1974), such as moths, beetles, wasps, and flies that all feed on or live within a plant. Releasing a single biocontrol agent may not have the same effects on its host plant compared to the full gamut of natural regulators that occur in the plant’s native environment.

Even though all invasive plants may have coevolved natural enemies that regulate their populations within their native range, approved biocontrol agents are available for only a few invasive plant species in the United States. The complex of natural enemies associated with the target plant in its native range may fail to provide suitable candidates for biocontrol for a number of reasons, such as failing to meet host-specificity standards. Likewise, some invasive plants are not suitable candidates for management with biocontrol (Wilson and McCaffrey 1999). For example, plants that are closely related to native, threatened and endangered, or economically important plant species are often poor candidates for biocontrol programs because of difficulty in finding natural enemies that will not damage the target plant’s close relatives.

Impacts of Biocontrol on Invasive Plant Populations

Once released, biocontrol insect populations typically require two to three years to successfully establish, and 10 to 20 years before they significantly affect the invasive plant population. As the direct and indirect effects of biocontrol agent attacks reduce the host invasive plant’s ability to compete within the plant community, invasive plant populations gradually decline, but are not eliminated. Biocontrol therefore has limited application for situations where rapid or complete invasive plant control is required. However, for widely established invasive plants, or for established plants with the potential to become widespread, biocontrol may be an appropriate strategy.

Purple loosestrife infestations (background) have been dramatically decreased since biocontrol beetles were released at Montezuma NWR in New York. Photo credit: J&K Hollingsworth/USFWS

For example, Montezuma NWR in New York, began using Galerucella spp. beetles in the mid-1990s to manage widespread stands of purple loosestrife (Lythrum salicaria) that infested nearly half of the refuge’s 3,200 acres. Although physical and chemical methods were effective for controlling small, localized infestations, these methods were too costly and environmentally degrading for sustained long-term management over large areas.

In areas where desirable vegetation is absent, suppression of one invasive plant species can lead to subsequent invasion and community dominance by another invasive plant. This phenomenon is referred to as the “biocontrol treadmill” (McCaffrey and Wilson 1994). When more desirable perennial vegetation is not available to fill niches opened up by the suppressed target species, another undesirable species for which biocontrols are not available may become established in its place. For example, Campbell and McCaffrey (1991) found that following successful suppression of St. Johnswort (Hypericum perforatum) in Idaho, the study sites reverted to the nonnative annual grass community that predated introduction of St. Johnswort. For proposed release sites in which multiple invasive species occur or are nearby, a plan for controlling these species is an integral part of the management approach.

The effects and effectiveness of biocontrol for managing invasive plant populations in general is highly variable and depends on the unique interactions between biocontrol agents and host plants, as well as a number of other biological, environmental, and procedural factors. A biocontrol agent that is very effective in controlling an invasive plant population under a given set of conditions may fail to establish, fail to reach populations levels to significantly reduce invasive plant populations, or (in rare cases) cause unanticipated negative non-target impacts under a different set of conditions.

Biocontrol Success on Refuges

The factors affecting biocontrol successes and failures are not fully understood and practitioners are continually gathering data to help improve the predictability of biocontrol effects. Although only about one-third of invasive plant biocontrol projects in the United States exhibit successful control (DeLoach 1991), there are a number of well-documented successes for biocontrol of several terrestrial and aquatic invasive plant species, including tansy ragwort (Senecio jacobaea), leafy spurge (Euphorbia esula), musk thistle (Carduus nutans), St. Johnswort (Hypericum perforatum), water hyacinth (Eichhornia crassipes), hydrilla (Hydrilla verticillata), and alligator weed (Alternanthera philoxeroides). Many National Wildlife Refuges have experienced significant success with biocontrol programs.

When effective, biocontrol can have a number of advantages over other invasive plant management methods (Culliney 2005):

Biocontrol agents can establish self-perpetuating populations and expand throughout the target invasive plant’s range, including areas with difficult access.

Regulation of the invasive plant population can be long-term with biocontrol; the densities of the biocontrol agent populations adjust themselves in response to changes in invasive plant density.

The impact of host-specific agents is focused on a single plant species, minimizing the likelihood of harm to other nontarget plants.

Overall, the cost of biocontrol is low relative to other approaches such as chemical and physical control, and expenses are incurred at the beginning of a program rather than on a continuing basis (not including the costs of long-term monitoring).

Effects of Biocontrol on the Environment

The two ecological cornerstones discussed above provide the framework for ensuring effectiveness and safety in classical biocontrol. Current protocols and regulations for selecting and importing biocontrol agents into the United States involve rigorous testing to ensure that biocontrols will not attack nontarget plant species. However, these protocols are not designed to thoroughly assess and predict the biocontrol agent’s long-term effectiveness and risks associated with complex ecological interactions.

Several authors have reviewed benefits and risks of biocontrol (Culliney 2005, Delfosse 2005, Simberloff and Stiling 1996, Hoddle 2002, Louda and Stiling 2004, and others). Assessment of the potential risk from biocontrol is not simple, and oversimplification can lead to erroneous conclusions (Delfosse 2005). Biocontrol does present some risk of unintended, adverse impacts and once biocontrol agents are released and established the biocontrol process cannot be reversed. The risk of using biocontrol must be weighed against its potential benefits, as well as the risks associated with other management methods, and the risk of doing nothing to manage invasive plant populations.

Biocontrol can be beneficial relative to other methods. However, lack of quantitative evidence regarding complex interactions and outcomes associated with introducing nonnative organisms warrants increased caution and continued development of improved ecological risk assessment (Louda and Stiling 2004). The following are a few examples of how biocontrols have been linked to nontarget impacts and unforeseen interactions.

Nontarget Attacks and Host-Shifting

Larvae of the seed head weevil Rhinocyllus conicus form galls in musk thistle (Carduus nutans) seed heads. Unfortunately, this biocontrol agent has also attacked nontarget, native thistles in North America. Photo credit: L Kok/VA Polytechnic Institute and State Univ., www.forestryimages.org

Substantial evidence of a biocontrol agent’s host-specificity is required prior to approval for release into the United States to protect nontarget plant species. However, host-specificity testing does not predict the potential extent of spatial range expansion or possibilities of genetic or behavioral changes that may occur as the introduced organism adapts to its new environment. Host-range expansion can occur (Secord and Karieva 1996, Hopper et al. 1993) and studies show unexpected levels of nontarget feeding and impact by some biocontrol agents (Howarth 1991, Follett and Duan 2000, Wajnberg et al. 2001).

Practitioners should be wary of using a biocontrol organism that fed on nontarget hosts during host-specificity testing, especially if there are closely related plant species in the proposed release area that are valued native plants, particularly plant species that are federally listed as endangered or threatened. Even if the biocontrol organism is unable to fully develop in or on nontarget hosts, feeding damage alone may be harmful to a threatened or endangered species population.

Accidental Introductions

The prickly pear cactus moth, Cactoblastis cactorum, was introduced to the Carribbean to control prickly pear cactus (Opuntia). The moth is now spreading on its own along the Gulf Coast of the United States and threatens native cacti species in the Southwest. Photo credit: I Baez/USDA ARS

Nonnative plant enemies can be unintentionally introduced to new areas via the same modes and pathways that facilitate introduction of nonnative invasive plants. Quarantine and screening protocols are designed to ensure that only intended organisms are imported into the United States and that they are free from biological contaminants.

Several invasive plant natural enemies have been introduced unintentionally to North America including the Canada thistle bud weevil (Larinus planus), the yellow starthistle-feeding false peacock fly (Chaetorellia succinea), the thistle tortoise beetle (Cassida rubiginosa), and the prickly pear moth (Cactoblastis cactorum). Accidental introductions of pathogens of insects can also occur. For example, the pathogen Nosema was accidentally introduced as a contaminant of a weevil (Trichosirocalus horridus) introduced to control musk thistle (Andres and Rees 1995).

Collecting biocontrol agents from one invasive plant population and releasing them into another may also present risk of accidental introductions. Invasive plant material containing biocontrol agents is sometimes used to transport biocontrol agents to other populations of the same invasive plant. To avoid introducing unapproved parasitoids and pathogens that may be associated with the plant material, plant materials can be held until the adult biocontrol agents emerge. Adults can then be collected and redistributed.

Ecological Replacement

Southwestern willow flycatcher.
Photo credit: USDA FS

Indirect nontarget effects can occur when a biocontrol is used to control invasive plant populations that have become integrated into the native community by physically or functionally replacing a native plant species. For this reason, a proposed biocontrol agent of saltcedar (Tamarix spp.) in the southwestern United States was initially rejected because of risks to the federally listed, endangered southwestern willow flycatcher (Empidonax traillii extimus), which currently relies on saltcedar trees for nesting sites in areas where saltcedar has replaced its native nesting habitat (USFWS 1993). The concern was that biocontrol of saltcedar would remove nesting habitat before native vegetation would be restored for the bird species. As with any management method, the potential consequences of controlling an invasive plant species is important to consider.

Researchers at New Mexico State University, with approval of USFWS, Southwest Region, are conducting a study on the effects of saltcedar leaf beetles released in a heavily restricted and geographically isolated area of the Tularosa Basin. These studies will provide data that can be used to evaluate how the leaf beetle might impact saltcedar and southwestern willow flycatcher habitat found along the Rio Grande River in New Mexico.

Food Web Interactions

If an established biocontrol agent is ineffective in reducing its host densities, population levels of the biocontrol agent may remain abnormally high. Nonnative organisms may cause unpredictable and potentially significant indirect effects for native species through food webs. In recent studies, knapweed gall flies Urophora spp. established and remained host-specific, but failed to control populations of their knapweed host plants. As a result, the biocontrol agents became superabundant. The gall flies were consumed by native deer mice (Peromyscus maniculatus), comprising 85% of their diet. The resulting deer mice population increase of two- to three-fold may have ramifications in terms of deer mice feeding on native plants and serving as vectors of hantavirus (Pearson et al. 2000, Ortega et al. 2004, Pearson and Callaway 2005).